Goodwill and Intangible Assets - Changes to the Carrying Amount of Goodwill by Segment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Roll Forward]
Beginning Balance	$ 2,930,211,000	$ 2,951,317,000
Additions due to acquisition	4,176,000	4,617,000
Reclassified as held-for-sale		(16,000,000)
Foreign currency translation impact	(2,873,000)	(9,723,000)
Ending Balance	2,937,260,000	2,930,211,000
Accumulated goodwill impairment losses	0
Research
Goodwill [Roll Forward]
Beginning Balance	2,651,193,000	2,670,934,000
Additions due to acquisition	4,176,000	4,617,000
Reclassified as held-for-sale		(16,000,000)
Foreign currency translation impact	(2,180,000)	(8,358,000)
Ending Balance	2,657,549,000	2,651,193,000
Conferences
Goodwill [Roll Forward]
Beginning Balance	183,951,000	184,021,000
Additions due to acquisition	0	0
Reclassified as held-for-sale		0
Foreign currency translation impact	(46,000)	(70,000)
Ending Balance	183,997,000	183,951,000
Consulting
Goodwill [Roll Forward]
Beginning Balance	95,067,000	96,362,000
Additions due to acquisition	0	0
Reclassified as held-for-sale		0
Foreign currency translation impact	(647,000)	(1,295,000)
Ending Balance	$ 95,714,000	$ 95,067,000